1. Field of the Invention
The present invention relates to a technique for forming interconnects of a semiconductor device, and more particularly to a substrate plating method and apparatus suited to fill a metal, such as copper (Cu), into recesses (e.g., trenches) for interconnects, formed on a semiconductor substrate.
2. Description of the Related Art
Conventional integrated circuits (ICs), which employ two-dimensional packaging of circuits onto a semiconductor substrate, have increased the integration degree by making circuits finer. The current circuit design rule is already in the 90 nm generation, and the 45 nm design rule is in a developmental stage when finer circuits are becoming difficult with two-dimensional packaging of circuits. In order to further increase the degree of integration, studies have been made actively on three-dimensional packaging which involves the lamination of a plurality of semiconductor substrates and the formation of interconnects that penetrate the laminate of semiconductor substrates.
A damascene process is currently used widely for the formation of copper interconnects in a semiconductor substrate. The damascene process comprises filling interconnect trenches formed in a semiconductor substrate (Si wafer) with copper, and removing extra deposited copper, e.g., by CMP (chemical mechanical polishing) to form copper interconnects in the trenches. Electroplating is mainly used for the filling of copper because preferential progress of plating from the bottoms of trenches becomes possible by carrying out electroplating using a plating solution which is an acidic copper sulfate solution containing certain types of additives. The additives generally include an inhibitor based on PEG (polyethylene glycol), a plating accelerator based on SPS [bis(3-sulfopropyl)disulfide], a leveler and chloride ion (Cl−).
When carrying out plating by bringing a surface into contact with a plating solution containing PEG and Cl−, the plating surface is basically in a plating-inhibited condition due to adsorption of PEG and Cl− onto the surface. A plating accelerator, such as SPS, when added to the plating solution, is considered to be adsorbed onto the plating surface upon plating and weakens the plating inhibiting effect of PEG and Cl−, thereby accelerating the progress of plating. As shown in FIG. 4, as plating progresses, the surface area decreases in the bottom corners 23 of a trench 21 formed in a semiconductor substrate (Si wafer) 20, whereby a plating accelerator, having a strong property of remaining on a surface, becomes condensed to increase its coverage. This may explain preferential progress of electrodeposition in the bottom of the trench 21.
A plating solution also containing a leveler is widely used in actual copper plating. Unlike PEG, a leveler by itself adheres to a plating surface and strongly inhibits plating. The leveler that has been adsorbed onto the plating surface is considered to be consumed with the progress of plating either by being taken into the copper plated film or by decomposition. Accordingly, the concentration of the leveler in the plating solution, which has intruded into the depth of a recess such as the trench 21, decreases by a diffusion-controlling mechanism. Thus, the leveler is adsorbed onto a plating surface in a high amount on the outer surface where the plating solution having a high leveler concentration is present, thus strongly inhibiting plating. On the other hand, in a recess, especially in its deeper portion, the leveler concentration of the plating solution decreases and adsorption of the leveler onto a plating surface decreases, resulting in weaker inhibition of plating. Progress of plating from the depth of the trench 21 can thus be expected. It is the current general view that the use of a plating solution containing such additives is essential to filling of a metal by electroplating into large trenches for three-dimensional packaging.
As described above, in the bottom-up metal-filling mechanism, a plating accelerator in a plating solution becomes condensed in the bottom corners 23, thus accelerating plating in the bottom. Though an amount of a plating accelerator adsorbed on a plating surface (or surface to be plated) is small on or shortly after immersion of the surface in the plating solution, because of the accelerator's strong property of remaining on the plating surface, the amount of the plating accelerator adsorbed on the plating surface gradually increases with the progress of plating. Thus, the adsorption reaches saturation in due course when the plating accelerator is adsorbed in a considerable amount on the entire plating surface irrespective of the surface configuration. For example, when carrying out plating at a current density of about 100 A/m2, the increase in the amount of a plating accelerator adsorbed on a plating surface will almost come to saturation after about a 10-minute period of plating.
Trenches for forming interconnects in a semiconductor substrate have a width dimension of several μm to several tens nm and a depth dimension of about 1 μm, whereas trenches for use in three-dimensional packaging, on the other hand, have a width dimension of 10 to 20 μm and a depth dimension of 50 to 100 μm, and thus is two orders of multitude larger than the former trenches. It is difficult to carry out plating of a substrate surface having such large trenches in a bottom-up manner because of the following two main reasons:
(1) In the case of trenches for interconnects in a semiconductor substrate, filling of a metal into the trenches by electroplating is generally completed within several minutes. Therefore adsorption of a plating accelerator onto a plating surface does not reach saturation, thus not causing any problem associated with saturation of the adsorption. In the case of large trenches for three-dimensional packaging, on the other hand, it can take several hours to fill a metal into the trenches by electroplating. Accordingly, adsorption of a plating accelerator in a plating solution onto a plating surface reaches saturation, when the plating accelerator is adsorbed on the entire plating surface. Thus, the plating accelerator has been condensed in the bottom corners 23 shown in FIG. 4, thereby accelerating plating in the bottom portions of the trench. At the same time, a considerable amount of the plating accelerator is adsorbed also on the other portion of the plating surface than the bottom corners 23. There is therefore no significant difference in the plating rate between the bottom corners 23 and the other portion.
(2) In the case of large trenches for three-dimensional packaging, the trench 21 shown in FIG. 4 is deep. Therefore, the concentration of copper ions in a plating solution decreases in the deep portion of the trench 21 because of diffusion-controlling mechanism. Accordingly, even if there is a sufficient effect of the plating accelerator, the plating rate is low in the bottom of the trench 21 due to an insufficient supply of copper ions.
Though filling of copper into trenches has been achieved by electroplating using a plating solution comprising an acidic copper sulfate solution containing the above-described additives, the plating takes a considerable amount of time and, in addition, control of such a plating bath necessitates a complicated operation (see Japanese Patent Laid-Open Publication No. 2003-328180).